A traditional display system using a light emission type element as a light source or what is called an active display such as a cathode-ray tube (CRT), liquid crystal display (LCD), plasma display panel (PDP), electroluminescence display (ELD) etc. has characteristics of being bright and easy to see. However, these various displays have such problems as causing visual weariness when operating for a long time due to looking directly at the light emission type element or a light source. Further, mobile equipment such as mobile phones etc. are often used outdoors. They have also such a problem that visibility goes down under the sun light because light emission is offset. Demand for LCDs, which is a light emission type element, is expanding briskly and they are used for various large and small-sized displays. However, LCDs have such a problem of a narrow viewing angle, a problem to be improved in terms of viewability when compared with other light emission type elements.
Recently, demand for reflection type displays, which can display brightly in full color with excellent color purity and low power consumption, has been increasing.
In the meantime, with the popularization of computers, the amount of paper used for communication and storing of documents has been decreasing. However, there is still a strong tendency to print out digital information on paper and read it. Accordingly, the amount of paper scrapped after temporary use is increasing these days. In addition, the amount of paper that is daily consumed for printed books, magazines, newspapers, etc. is seen as a threat in terms of securing of resources and preservation of environment. As far as the medium is not changed, there is no prospect of decreasing the amount of paper scrapped. However, given the way of information recognition and the way of thinking by the human being, the superiority of “paper” over “display”, such as a CRT or a transmission type liquid crystal display, cannot be ignored.
Therefore, development of an electronic paper or electronic medium, in which the merit of paper and the merit of displays are combined together, is expected. The electronic medium may be used as an alternative to paper. As desired characteristics for the electronic paper, being a reflection type display element, having high white reflectance and high contrast ratio, being capable of displaying with high definition, having memory effect in display, being capable of driving with a low voltage, being thin and light, being inexpensive, etc. can be exemplified.
As the display method of electronic paper, a reflection type liquid crystal method, an electrophoresis method, a two color ball method, an electrochromic (EC) method, etc. can be exemplified.
As the reflection type liquid crystal method, a G-H type liquid crystal method using dichromatic pigment, a cholesteric liquid crystal method, etc. can be exemplified. This reflection type liquid crystal method does not use a backlight so that it consumes small electric power when compared with the light emission type liquid crystal method. However this method has a view angle dependency and low light reflectivity, thereby has a problem that the screen inevitably becomes darker.
In the electrophoresis method which utilizes electrophoresis phenomenon, white pigment, black toner or the like is moved onto an electrode by the effect of an electric field. The two-color ball method comprises a spherical body painted with two colors such as white and black in a half-and-half manner. In this case, rotation by the effect of an electric field is utilized. Both methods have a merit that they consume low electric power and that they have no view angle dependency. However, in these methods, it is necessary to have enough gaps into which granular bodies can enter but closest packing is difficult to be achieved so that it is difficult to realize a high contrast display. In a case of full-color display, a juxtaposition mixture method using a color filter is applied, therefore there are problems such as a decrease in reflectivity and an inevitable decrease in brightness of the screen.
On the other hand, in the EC system, a color developing and reducing phenomenon is utilized, which appears together with a reversible oxidation-reduction reaction when an electric field is applied. The EC display element has been used widely in light control mirrors of automobiles, clocks, etc. The display having this EC display element, does not require polarization plate, etc., has no view angle dependency, is a light receiving type and accordingly has high visibility, has a simple structure and can be made large in size. Furthermore, this display can develop various color tones by selecting suitable material.
To display in full color using EC display element, it has been known that pigments capable of developing colors such as cyan (hereinafter, simply called C), magenta (hereinafter, simply called M), and Yellow (hereinafter, simply called Y), which are used in subtractive color mixture, are applied to stack the C, M, Y color-development layers to form a juxtaposition alignment or lamination alignment. Thereby, a display device capable of developing a full color is obtained. For example, black color can be displayed by mixing colors of C, M and Y, while white color can be displayed by bringing the pigments into colorless and transparent state, and also by making the background color white. As described above, the EC display elements are reflection type display elements in which color development/color reduction can be electrically repeated, they are superior to other display system in terms of burden to eyes and in terms of contrast.
As material for color development layer, a π-electron based conjugated polymer is publicly known. This π-electron based conjugated polymer is classified into various sort of polymers such as polypyrrole, polyaniline, polyparaphenyl vinylene, polythiophene, etc. and has great potential as material for polymer light emitting diodes, thin film displays, solid-state illumination, organic photocells, memory devices, organic field effect transistors, printed electronics, conductors, LASER, sensors, solid condensers, etc. Among this π-electron-based conjugated polymers, some that exhibit electrochromic characteristics have been known. To obtain an EC display element that can display in full color by development/reduction of the color of C, M and Y, it is necessary that the electrochromic characteristics of the π-electron-based conjugated polymers do a change from a color-developed state of C, M and Y to a colorless state respectively. However, the electrochromic characteristics of almost all of the general π-electron-based conjugated polymers do a color change between a color developed state to another color developed state, but material that exhibits color change from a color-developed state to colorless state is extremely rare.
As a typical material that exhibits a color change from a color developed state to a near colorless state, poly(ethylene-3,4-dioxythiophene) have been known so far. However, this material is a π-electron based conjugated polymer that does a color change from a color developed state having a dark blue color near to C color to a reduced state having a light blue color. No material has been known that does a color change from the color M or Y to a colorless state.
Japanese Patent Publication 2009-501240 and Jung Youl Lee et al., Polymer Preprints, 2003, vol. 44, No. 1 p. 1163 disclose a polymer having a structural unit of 2-alkylthieno[3,4-d][1,3]thiazole etc., a copolymer having a structural unit of thiophene, etc., and a method for producing a monomer compound of 2-nonylthieno[3,4-d][1,3]thiazole and its polymer. However, there were no disclosure about a monomer compound in which two molecules of such a compound as 2-alkylthieno[3,4-d][1,3]thiazole and 2-nonylthieno[3,4-d][1,3]thiazole etc. are joined together with an aryl compound, etc., and there were no description about a polymer obtained by polymerizing these monomers or the electrochromic characteristics of this polymer.